The present invention is directed to prosthetic knee joints having femoral and tibial components.
Prosthetic knee joints have been disclosed in U.S. Pat. Nos. 4,298,992 and 5,702,458, on which the present inventor is named as a co-inventor.
The present invention is directed to a prosthetic knee joint which offers a greater degree of knee flexion over that which was available with prior art prostheses. This can be accomplished by placing the contact point between the tibial component and the femoral component proximate the posterior edge of the tibial component at the time that the femur, and hence the femoral component, is flexed to the highest possible angle of flexion. However, in providing for the highest possible angle of flexion, by positioning the contact point between the tibial component and the femoral component proximate the posterior edge of the tibial component, there is a possibility that mechanical function of the prosthesis may be compromised. The contact area between the femoral and tibial condyles may be diminished, thus increasing contact stress, and possibly joint wear. Also, at extreme flexion, the femoral component, at either the medial or lateral femoral condyle (or both), may lose contact with its tibial counterpart. This condition could occur if there is rotation of the tibia and femur relative to the other, in which case edge loading of the posterior edge of the load bearing tibial condyle might occur.
The knee joint prosthesis of the present invention includes an auxiliary pair of load bearing surfaces in addition to other load bearing surfaces of the tibial and femoral components, which are the medial tibial and femoral condyles, the lateral tibial and femoral condyles, and the central tibial and femoral cam surfaces. The auxiliary pair of load bearing surfaces are located, on the tibial component, posterior (i.e., on the rear side of the knee joint) to the central cam, centered between the tibial condyles, and on the femoral component, posterior to the femoral central camming surface, centered between the femoral condyles.
It is contemplated that under normal mechanical operation of the knee joint prosthesis of the present invention, that is, with an absence of surface wear or deformation and no rotation at high flexion angles (i.e., greater than 120 degrees, preferably greater than 125 degrees), the auxiliary load bearing surfaces will not transmit loads, or will transmit only a minimal load. However, when wear occurs to an appreciable degree, or where there is internal/external rotation between the tibial and femoral components at high flexion angles, the tibial and femoral auxiliary load bearing surfaces come into contact with each other, transmitting a load in order to stabilize the joint.
If the knee is moved to a high flexion angle, and then experiences rotation between the femur and tibia, the movement of one femoral condyle off of the posterior edge of the corresponding tibial condyle results in contact and load bearing between the auxiliary load bearing surface pair. This prevents excessive vargus/valgus joint motion, thus stabilizing the knee joint and effectively transmitting the joint load.
The relatively smaller contact surface experienced by the knee joint at high flexion angles may result in joint surface wear or cold flow of the joint surfaces. This can produce a decrease of bearing thickness. If the thickness of the medial or lateral tibial condyles decreases, then the auxiliary bearing surface pair would come into contact and transmit a portion of the joint load.
The auxiliary surface of the femoral cam may also increase the xe2x80x9cjumping heightxe2x80x9d of the femur with respect to the tibia. Jumping height is the term used to describe the amount of separation required between the normally contacting tibia and femur that would allow the femur to move forward and jump over the tibial cam. This occurrence, called dislocation, is undesirable. Therefore, maximization of jumping height is desirable in order to prevent dislocation at high flexion angles. Since, during high angles of flexion, the auxiliary load bearing surface of the femoral cam remains proximally within the posterior boundaries of the articulating surfaces of the components, jumping height is maximized.